Method of producing a drawing and deep drawing steel resistant to ageing, particularly for single-coat enamelling

ABSTRACT

Process for producing deep drawing steel resistant to ageing includes reducing the carbon content of a steel melt having the composition 0.1% maximum carbon, manganese below 0.45%, 0.03% maximum phosphorus, 0.035% maximum sulphur and the balance iron to a maximum of 0.03% by vacuum treatment, adding manganese to the steel melt so that the maximum total manganese therein is 0.45%, and adding to the steel melt from 0.05 to 0.2 kg boron per ton of the steel melt.

United States Patent 11 1 I Riittiger METHOD OF PRODUCING A DRAWING AND DEEP DRAWING STEEL RESISTANT TO AGEING, PARTICULARLY FOR SINGLE-COAT ENAMELLING [75] Inventor: Karl Riittiger, Dortmund, Germany [73] Assignee: Dortmund-Herder Huttenunion Aktiengesellschaft, Dortmund, Germany 22 Filed: Feb. 5, 1971 21 Appl. No.: 113,075

Related US. Application Data [63] Continuation of Ser. No. 620,208, Feb. 24, 1967,

abandoned.

[30] Foreign Application Priority Data 1111 3,792,999 1451 Feb. 19, 1974 2,771,651 11/1956 Morgan et al 75/58 UX 3,026,195 3/1962 Edstrom eta]. 75/49 3,417,463 12/1968 Knuppel et a] 75/49 UX 2,999,749 9/1961 Saunders et al. 75/58 2,956,906 10/1960 Blickwede et a1. 117/129 2,144,200 l/l939 Rohn et a]. 75/49 X 2,093,666 9/1937 Vogt 75/49 X 3,512,957 5/1970 Brotzmann et al. 75/49 3,183,078 5/1965 Ohtake et a1. 75/49 3,208,844 9/1965 Kato et a1. 75/49 X 3,239,390 3/1966 Matsukura et al... 148/12.1 3,459,537 8/1969 Hornak 75 49 3,522,110 7/1970 Shimizu et a]. 148/12 Primary Examiner-Hyland Bizot Assistant Examiner-Peter D. Rosenberg Attorney, Agent, or Firm-Herbert L. Lerner, Esq.

[5 7 ABSTRACT melt.

6 Claims, No Drawings METHOD OF PRODUCING A DRAWING AND DEEP DRAWING STEEL RESISTANT TO AGEING, PARTICULARLY FOR SINGLE-COAT ENAMELLING This application is a continuation of our application Ser. No. 620,208, filed Feb. 24, 1967 and now abandoned.

It is known for deep drawing steels resistant to ageing and cast in the killed condition and also deep drawing steels not resistant to ageing and cast in the unkilled condition to be used for the production of this sheet. When killed with aluminium however, deep drawing steels resistant to ageing have the'disagreeable property that the aluminium combines primarily with the oxygen and only secondarily with the nitrogen and that after the casting the resulting alumina particles are concentrated near the surface of the ingot. Such concentrations naturally impair the quality of the surface of the finished rolled material to a considerable extent, so that it is usually indispensable to apply flame scarfing in order to remove the surface layer. Apart from the not inconsiderable cost of flame scarfmg, losses of material of up to about 5 percent are incurred through the removal of the surface layer, if a similar surface quality to that of a steel cast in the unkilled condition is to be achieved.

In contrast, steels cast in the unkilled condition have a relatively clean surface zone,so that flame scarfing of the ingots is generally not necessary, but they have the considerable disadvantage tht they are not resistant to ageing. In their case the technological properties vital to deep drawing become modified in course of time to such an extent that further drawing or pressing processing is frequently uneconomical or even completely impossible because of an excessively high proportion of rejects.

In order to avoid the difficulties mentioned above and to obtain not only a clean ingot surface but also a steel resisting ageing, the procedure has been adopted of casting the steel in the unkilled condition and killing .it only in the ingot mould, that is to say adding the nitride forming substances which are important to its resistance to ageing, namely aluminium, titanium, zirconium, and boron, only when it is in the mould. Although this process is relatively simple to carry out, the gravity of the sheet nevertheless does not comply with the strict requirements, since the thin rolled sheet frequently exhibits streaks which originate from the solidification behaviour of steel killed in the unkilled condition and deoxidised in the mould, and which so far it has not been possible to eliminate.

The nitride forming substances which have an affinity for oxygen not only combine with the nitrogen, but also react primarily with the oxygen in the unkilled steel, so that when the latter solidifies in the mould there is a reduction of boiling reaction. The absence of an adequate boiling reaction in turn leads to the formation of so-called peripheral blow holes, which likewise constitute undesirable surface deflects. If however it is attempted to reduce the formation of peripheral blow holes or to promote the boiling by adding a smaller quantity of the nitride forming substances having an affinity for oxygen, the amount of the respective nitride forming substances, particularly boron, still remaining free after the deoxidation of the steel is no longer sufficient to make the steel resistant to ageing.

In order to eliminate peripheral blow holes, it has also already been attempted to reduce the oxygen content of the steel to such an extent that the reaction between the carbon and the oxygen in the steel, which was the cause of the formation of peripheral blow holes, is practically suppressed. Since the reduction of the oxygen content through deoxidation with aluminium in excess is unacceptable for the reasons explained above i.e., because of the resulting concentration of alumina particles on the surface of the ingot, it has finally been attempted to produce a semi-killed steel by adding a deficient quantity of aluminium. This semikilled steel has so low a free oxygen content that its ingot structure corresponds approximately to that of :1 killed steel. This process however can scarcely be carried out with the necessary degree of safety on a large industrial scale, since the oxygen content must be adjusted extremely accurately, quite apart from the fact that even small additions of aluminium lead to the formation of the undesirable alumina particles.

The aim underlying the present invention consists in eliminating the abovedescribed disadvantages and providing a process which furnishes a high yield of a steel resisting ageing and have a good surface quality. The invention is based on the surprising discovery that even steels having a free oxygen content is excess of the maximum limit applied hitherto can be made resistant to ageing by a suitable addition of boron and at the same time will solidify in the fully killed condition if the carbon content is reduced to a low value preventing the formation of peripheral blow holes.

According to the invention it is therefore proposed to reduce the carbon content of a steel containing a maximum of 0.10% of carbon, up to 0.45% of manganese, a maximum of 0.03% of phosporus, and at most 0.035% of sulphur, by vacuum treatment, to below 0.03%, preferably to 0.01%, then bringing the manganese content up to 0.45%, and adding up to 0.2 kg of boron per ton of steel to the melt.

With a relatively high oxygen content, the decarburisation of the steel by the vacuum treatment is a feature which is essential to the suppression of the boiling reaction in the mould. The vacuum decarburisation may be assisted by adding ore or other oxygen compounds. Instead of iron ore, other solid or gaseous oxidising agents may also be introduced into the melt.

With the exception of the very low carbon content, the composition of the steel produced by the method of the invention corresponds to the usual deep drawing grades cast in the unkilled condition, while the very low carbon content of under 0.03% does not appreciably affect the technological properties of the steel or of the steel or of the thin sheet. Moreover, the steel treated by the process of the invention exhibits the solidifaction behaviour of a killed steel and in particular is free from peripheral blow holes, since the carbon content responsible for the formation of the latter is practically removed by the vacuum treatment. In the presence of a small quantity of free boron which is still present after the combination of the major part of the nitrogen, the steel is resistant to ageing. Experiments have moreover shown that the oxygen content may vary within relatively wide limits without thereby impairing the solidifi cation structure of the ingot and the resistance of the steel to ageing.

In a trial melt a steel melted by the oxygen blast method and containing 0.04% of carbon, 0. 12% of manganese, 0.02% of phosphorous, 0.02% of sulphur, and 0.04% of nitrogen was subjected to vacuum treatment by the known vacuum syphon method. In order to be able to reduce the carbon content of the melt to a sufficient extent, a total of 1 kg (per ton) of hematite, per ton of melt was added to the melt in five portions.

a cold working step giving a reduction in cross sectional area f42.8%, following this skin pass rolling with a tinishing rolling step giving a further reduction and after annealing at 680 690C, both in the aged and in the normal condition. The data obtained are listed in Table 1 below.

Table 1 Erichsen Aged (2 hours Value at 70C) Al Sample Elastic Ultimate Elong Elong- Position Limit Stress ation Requi Acnb Elastic Ul i ation (kP/mml) red al Limit Stress Head 20.8 32.1 42.5 11.44 12.76 20.8 31.8 41.3 0.052 25% from head 20.3 31.5 42.5 11.60 12.93 20.5 31.4 40.0 0.045 Centre 20.1 31.4 42.5 11.57 13.03 20.1 31.3 42.5 0.023 25% from foot 19.9 30.9 44.3 11.60 12.96 19.8 31.2 42.5 0.018 Foot 20.0 31.6 H 41.3 11.60 12.66 19.0 31.4 40.0 0.017

e degasification of the 150 ton melt was substantially completed after vacuum treatment for minutes, the final carbon content being 0.01 1% and the oxygen content 0.019%. After the vacuum treatment. 270 kg of manganese in the form of super-refined ferromanganese and 22.5 kg of boron in the form of ferroboron were introduced into the vacuum vessel. Thereupon the steel, containing 0.01 1% of carbon, 0.28% of manganese, 0.02% of phosphorus, 0.02% of sulphur, 0.004% of nitrogen and 0.009% of boron was cast into slabs.

The slabs were rolled out into hot rolled strip of a thickness of 2 mm and then into cold rolled strip of a thickness of 0.8 mm. Thereupon the strip was subjected to subcritical annealing at 680C and re-rolled on a skin pass table with a thickness decrease of 1.0%. The yield point of samples taken from the strip after the skin pass rolling was 17.9 19.2 kg per square mm, individual values being still lower. In addition, the stress and strain curves in the tensile test showed no pronounced yield point. The samples were also resistant to ageing, since after ageing for 8 weeks at room temperature it was not possible to detect any rise in the yield point. The tensile tests with artifically aged samples also showed no pronounced yield point.

In the course of a second trial melt, a conventional steel, likewise produced by the oxygen blast method, was subjected to vacuum treatment, after which it had the following composition 0.016% carbon 0.33 manganese 0.01 1% phosphorus 0.032% sulphur 0.004% nitrogen.

This steel contained neither silicon nor aluminium. It was cast in a -10 ton ingot mould having a base crosssection of 1,400 X 640 mm, a height of 1,900 mm, and a conicity of'2.l%, 800 grams of aluminium granules per ton of melt being added to the steel during pouring and about 30 seconds before the 10 ton mark was reached. it was found that the first material produced from this steel did not require flame scarfing and that Ladle 11 the surface of the thin sheet produced from it was completely free from defects.

The technological properties of the steel produced by the method of the invention were determined from a cold-rolled strip of a thickness of 2 mm, produced by In a third test, a steel of usual deep drawing quality was produced in an open-hearth furnace, its carbon content amounting to 0.04%, which was reduced to 0.010% by vacuum treatment. After vacuum treatment the steel of a ladle l was poured immediately, while boron was added at the end of the vacuum treatment to the steel of a ladle ll. Steels l and 11 contained 0.01 1% of carbon, 0.01% of silicon, 0.32% of manganese, 0.018% of phosphorous, 0.022% of sulphur, and steel 11 contained 0.01% of boron.

Both steels were cast into slabs and rolled out in the.

usual manner into hot rolled strip and then into cold rolled strip. After sub-critical annealing and'skin pass rolling, single-coat enamelling tests were carried out on the cold rolled strip of a thickness of 0.4 mm, the results of which are shown in Table 11 below.

Table 11 Adhesiveness Surface quality Ladle 1 poor very good to good poor very good From the test data shown in Tables 1 and II it can be seen that the steel treated in accordance with the invention is resistant to ageing, has excellent surface quality, and in consequence of the addition of boron is particularly suitable for single-coat enamelling.

I claim:

1. Process for producing deep drawing steel resistant to ageing, which comprises sequentially reducing. the carbon content of a steel melt, that has the composition 0.1% maximum carbon, manganese below 0.45%,

0.03% maximum phosphorus, 0.035% maximum sulphur and the balance iron, to a maximum carbon content of 0.03% by vacuum treatment, adding manganese to the steel melt so that the manganese content therein is 0.45%, and adding to the manganese-treated steel melt from 0.05 to 0.2 kg boron per ton of the steel melt.

2. Process according to claim 1, which further comprises adding to the steel melt treated with the manganese and boron from 0.2 to 0.8 kg aluminum per ton of the melt.

3. A process as claimed in claim 2, in which the carbon content of the melt is reduced to 0.01% by vacuum treatment.

pound is iron ore.

6. A process as claimed in claim 5, comprising the further operation of adding gaseous oxidizing agents to the melt during the vacuum treatment.

I STAG. 13L. ZPATEIIT FFICE I (TIQCFTTTTUF .1 n

. iuAle-b Or \JORLECTION Patent No. 3,79 ,999 Dated February 19, 1974 II Invent-012(8) KARL RUT'I'IGER It is certified that error appears in. the above-identified pater: and, that said Letters Patent are hereby corrected-as shown below In the heading t6 theprinted spec'ificationQ l-ine 15 "Feb. .26, 1966 Germany. 11945718" should read F b 26 1966. 7 Germany .D 49 157' V:Ia/18b 11ne l6, "Apr. 29, 1966 Germany...,... -L9981l8" should read 9 -Apr-. 129, 1966 9 Germany. .D 49 9 81 VIa/18 b line 17 Y'Oct. -18, 1966 Germany. 51335 10" should read I Oct. 18, 1966 Germany. .D 51 335 VIa/MOb Signed and sealed this 9th day of July I (SEAL) Attest: v MCCOY M. GIBSON, JR. c. MARSHALL DANN Attesting Officer V Commissioner of Patents 

2. Process according to claim 1, which further comprises adding to the steel melt treated with the manganese and boron from 0.2 to 0.8 kg aluminum per ton of the melt.
 3. A process as claimed in claim 2, in which the carbon content of the melt is reduced to 0.01% by vacuum treatment.
 4. A process as claimed in claim 3, comprising the further operation of adding at least one solid oxygenated compound to the melt during the vacuum treatment.
 5. A process as claimed in claim 4, wherein the compound is iron ore.
 6. A process as claimed in claim 5, comprising the further operation of adding gaseous oxidizing agents to the melt during the vacuum treatment. 